1. Field of the Invention
The present invention is related to a production method in which highly concentrated hydrate is produced efficiently by reacting hydrate producing substance, such as methane with water, and a device which is suitable for carrying out the same.
This application is based on Japanese Patent Applications Nos. Hei 11-69291 and Hei 11-69294, the contents of which are incorporated herein by reference.
2. Description of the Related Art
It is well known that a large amount of natural gas components, such as methane etc. are presented as hydrates under the ground of cold districts. The hydrates are existed stably at low temperatures and high pressures; therefore, they are anticipated to be natural gas sources for the next generation. In particular, a hydrate comprising methane (below simply denotes methane hydrate) is one kind of clathrate compound in which a methane molecule is situated into a cluster comprising water molecules stereoscopically positioned. The distance between methane molecules in the hydrate clusters is shorter than the distance between methane molecules in a gas cylinder under high pressure. That is, the methane molecules in a hydrate state are situated closely. Therefore, it is anticipated that to storage and transport methane in a methane hydrate state. In addition, the reaction between methane and water has a reversible equilibrium, and generates a large amount of hydration heat. Therefore, applications of a methane hydrate for a heat storage material, refrigerator, heat pump, etc. are currently being investigated.
As described above, many applications of the methane hydrate are anticipated, and therefore an investigation of synthesizing methane hydrate with a high efficiently is carried out, in addition to depending on the natural resources. However, in general, the pressure at which the methane hydrate is stabilized at 15xc2x0 C. is 100 kg/cm2 or greater. That is, methane hydrate is stabilized under conditions of low temperatures and high pressure; therefore, it is difficult to handle the methane hydrate. The handling the methane hydrate under such conditions is difficult. In order to solve the problem, many kinds of stabilizers for shifting the formation equilibrium conditions of the methane hydrate to the conditions of high temperatures and low pressures, have been investigated. As a result, it has been discovered that for example, aliphatic amines such as isobuthyl amine, isopropyl amine, etc. (Japanese Patent Publication, Second Publication No. Sho 53-1508 (Koukoku)), 1-3-dioxysolane, cyclobutanone, tetrahydrofuran, cyclopentanone, acetone, etc. (Seiichi Yokoi and others, Nippon Kagaku Bulletin, 1993 (4), page 378 to 394) are useful as stabilizers.
The production method in which hydrate is produced by spraying water into a gaseous phase of ethane which is one of the hydrate producing substance, thereby contacting ethane and water with a large contact area, has been suggested (INTERNATIONAL CONFERENCE ON NATURAL GAS HYDRATES (JUN. 2-6, 1996 TOULOUSE FRANCE).
In general, a device shown in FIG. 9, for example, has been used to produce methane hydrate using the above-mentioned production method. In FIG. 9, the synthesis device for methane hydrate comprises a pressure vessel 150 equipped with an aqueous phase injection pipe 151, a methane gas injection pipe 152, an outlet 153, and an agitator 154. The pressure vessel 150 is put into a thermostatic bath 155. Furthermore, the synthesis device comprises thermometers for measuring temperatures T1 and T2 in a gaseous phase and an aqueous phase in the pressure vessel 150, a pressure meter for measuring pressure P in the pressure vessel 150, an instrument for measuring a rotational frequency R of the agitator 154, and a thermometer for measuring temperature T3 in the thermostatic bath 155.
In order to synthesize the methane hydrate using the synthesis device, for example, an air in the pressure vessel 150 is expelled by introducing a methane gas in the pressure vessel 150 from the methane gas injection pipe 152. Then, an aqueous solution containing the stabilizer having a desired concentration is introduced into the pressure vessel 150 from the aqueous phase injection pipe 151 as an aqueous phase. The temperature of the aqueous phase in the pressure vessel 150 is set at the desired temperature by the thermostatic bath 155. Methane gas is introduced into the pressure vessel 150 from the methane gas injection pipe 152 while stirring with the agitator 154 until the pressure in the pressure vessel 150 reaches a desired pressure. When the stirring is carried out keeping these conditions, a hydration reaction occurs, and the pressure P in the pressure vessel 150 decreases. In addition, the temperature T2 of the aqueous phase rises due to a heat of hydration. The synthesis device is left alone until the temperatures T1 and T2 of the gaseous phase and the aqueous phase which are enclosed by the thermostatic bath 155 are substantially equal, while the pressure P in the pressure vessel 150 is adjusted by exhausting a part of the methane gas from the outlet 153, if necessary. Then, methane hydrate having a formation equilibrium pressure P at the temperature T2 can be obtained.
However, the conventional methane hydrate production method using the production device shown in FIG. 9 has following problems. The reaction between methane and water is carried out due to an absorption of methane gas into the aqueous phase at a gas-liquid interface. As shown in FIG. 10, the density of the methane hydrate MH produced by the reaction is smaller than the density of water (the theoretical density of methane hydrate is 0.915 g/cm2). Therefore, the methane hydrate MH comes near the surface of a liquid phase (aqueous phase) L, and forms a methane hydrate layer. The adsorption of methane M at the surface between a gaseous phase G and a liquid phase is prevented by the methane hydrate layer. In addition, the viscosity of the liquid phase L increases, depending on the production degree of the methane hydrate, and the stirring effect of the liquid phase L is insufficient. Consequently, it is difficult to produce the methane hydrate having a high concentration.
In addition, the concentration of the methane hydrate in the liquid phase L increases, depending on an amount of methane gas injected from the methane gas injection pipe 152. However, the ratio of water to the methane gas, which remains in the liquid phase L decreases, while the reaction is carried out. Then, the reaction reaches an equilibrium, and the hydration reaction does not proceeded. Therefore, from this point of view, it is also difficult to produce methane hydrate having a high concentration.
Furthermore, the period from the introduction of the aqueous solution into the pressure vessel 150 to the end of hydration reaction between the aqueous solution and the methane gas, is long. Namely, a long period to fix the temperature T2 of the aqueous solution at a desired temperature by the thermostatic bath 155, is necessary. Therefore, the production efficiency of the methane hydrate is low.
Water particles contact ethane with a large contact area in the production method in which water is sprayed in an ethane gaseous phase. However, there is the possibility that the produced hydrate adhering the surface on especially large water particle like an epidermis. The water particles enclosed by the, hydrate does not react with the ethane gas. From this point of view, there is still room for improvement of the production efficiency of the hydrate. In addition much time is necessary to reduce the temperature of the sprayed water into the reaction vessel to the temperature required to produce the hydrate. Therefore, there is also still room for improvement of the production efficiency of the hydrate.
Therefore, an object of the present invention is to provide a production method for hydrates in which the hydrate producing substance and water are reacted efficiently and highly concentrated hydrate is produced in a highly efficient and short period, and a production device suitable for promoting the production method.
According to a first aspect of the present invention, the present invention provide a production method for hydrate in which water and hydrate producing substance are reacted in a hydrate producing vessel comprising the steps of:
spraying water into a gaseous phase containing the hydrate producing substance in the hydrate producing vessel, thereby hydration reacting between water and the hydrate producing substance; and
supplying the hydrate producing substance in a gas state into an aqueous phase in the hydrate producing vessel.
According to the production method of the present invention, the temperature of the aqueous phase in the vessel is set in a range of 1 to 5xc2x0 C., for example, and the temperature of the aqueous phase is maintained in following processes. When the temperature of the aqueous phase is fixed at the required temperature, the hydrate producing substance, such as methane in a gas state is introduced into the aqueous phase from a lower part of the aqueous phase. Thereby, at least a part of the hydrate producing substance is absorbed by the aqueous phase from a gas-liquid interface, reacts with water, and changes to hydrate. The density of the hydrate produced by the reaction is smaller than the density of water. Therefore, the hydrate comes near the surface of the aqueous phase, accumulates at the surface of the aqueous phase, and forms a hydrate layer. Then, only hydrate layer is recovered. In this reaction system, the bubbles containing the hydrate producing substance rise continuously in the aqueous phase: therefore, the surface of the bubbles is not covered with hydrate having a highly density, and always contact water. As a result, the reaction between water and the hydrate producing substance is carried out efficiently. When the production method of the present invention is carried out stably and continuously, highly concentrated hydrate can be produced continuously and efficiently.
Non-reacted hydrate producing substance in a gas state which is not absorbed by the aqueous phase is effused from the top surface of the aqueous phase, accumulates at the top of the vessel, and form a gaseous phase. When water is sprayed into the gaseous phase, water and the hydrate producing substance in a gas state are contacted and hydrate is produced rapidly. In other words, the surface area per fixed volume of water increases by spraying. Then, the contact area between water and methane increases remarkably. Thereby, the formation rate of hydrate increases. The produced hydrate falls to the top surface of the aqueous phase, and derived, namely collected. Moreover, it is possible to recover water from the aqueous phase in the vessel and spray.
Furthermore, more rapid production of hydrate can be achieved by spraying supercooling water prepared in advance into the gaseous phase. That is, when supercooling water contacts the hydrate producing substance, the temperature of the hydrate production reaction system is reduced. Then, in order to maintain the thermal equilibrium of the reaction system, that is, to rise the temperature of the reaction system, the reaction between water and methane is promoted. In other words, in order to generate hydration heat, the hydration reaction rapidly occurs. Then, the methane hydrate can be rapidly produced. The supercooling water may be obtained by recovering water from the aqueous phase and cooling. Moreover, the same effect which is obtained by using supercooling water, can be obtained by increasing the pressure of hydrate production reaction system.
According to a second aspect of the present invention, the present invention provide a device for producing hydrate comprising:
a hydrate producing vessel,
a conditions adjusting device for adjusting the temperature and the pressure in the hydrate producing vessel to suitable conditions for producing hydrate,
a water supplying device for supplying water in the hydrate producing vessel and producing an aqueous phase,
a hydrate producing substance supplying device for supplying hydrate producing substance into the aqueous phase in the hydrate producing vessel and forming a gaseous phase containing hydrate producing substance,
a spraying device for spraying water into the gaseous phase in the hydrate producing vessel, and
a hydrate recovery device for recovering produced hydrate from the aqueous phase.
According to the device of the present invention, it is possible to carrying out the first production method easily and certainly. In addition, a spray nozzle or an ultrasonic vibration body can be used as the spraying device. The ultrasonic vibration body accumulates water and makes water particles fine by ultrasonic vibration. For example, the ultrasonic vibration body is preferably in a plate shape. When the ultrasonic vibration body is used as the spraying device, water particles can be divided more finely and uniformly. Then rapid production of hydrates can be achieved.
According to a third aspect of the present invention, the present invention provide a production method for hydrate in which water and hydrate producing substance are reacted in a hydrate producing vessel comprising the steps of:
ultrasonic vibrating a gaseous phase containing the hydrate producing substance and/or an aqueous phase in the hydrate producing vessel, thereby separating a produced hydrate coating adhering a surface on a water particle from the water particle; and
spraying water into a gaseous phase containing hydrate producing substance in the hydrate producing vessel, thereby hydration reacting with water and the hydrate producing substance.
According to the production method of the present invention, the surface area per fixed volume of water increases by spraying. Then, the contact area between water and the hydrate producing substance increases remarkably. Thereby, it is possible to increase the formation rate of hydrate.
Furthermore, there is the possibility that the produced hydrate adhering the surface on especially large water particle like an epidermis or a coating by the conventional production method. However, the epidermises (coating) comprising hydrate adhering the surface on especially large water particle are eliminated by vibrating the gaseous phase and/or the aqueous phase in the production method of the present invention. Then, water particle which is not already enclosed by hydrate can react with hydrate producing substance. Therefore, rapid production of hydrate can be achieved by the production method of the present invention.
According to a fourth aspect of the present invention, the present invention provide a device for producing hydrate comprising:
a hydrate producing vessel,
a conditions adjusting device for adjusting the temperature and the pressure in the hydrate producing vessel to suitable conditions for producing hydrates,
a hydrate producing substance supplying device for supplying hydrate producing substance into the hydrate producing vessel and forming a gaseous phase containing hydrate producing substance,
a spraying device for spraying water into the gaseous phase,
a hydrate coating eliminating device for ultrasonic vibrating the gaseous phase and/or the aqueous phase in the hydrate producing vessel,
a hydrate recovery device for recovery hydrate from the aqueous phase.
According to the device of the present invention, it is possible to carrying out the production method easily and certainly. In addition, an ultrasonic vibration generator provided with the outer wall of the vessel or the inner wall of the vessel, can be used as the hydrate eliminating device. Furthermore, the shape of the ultrasonic vibration generator may be a plate shape suitable for providing to the vessel wall or a net shape in which water particles can pass through.